(1) Field of the Invention
The present invention relates to a method and apparatus for mass-producing high quality oxide type single crystal films by a liquid phase epitaxial process.
(2) Related Art Statement
A single crystal of lithium niobate (LiNbO.sub.3) and a single crystal of lithium tantalate have been expected as optoelectronic materials. It is known that a thin film of lithium niobate may be formed on a substrate made of a lithium niobate single crystal or the like by the liquid phase epitaxial process. For instance, according to "Appl. Phys. Letters", Vol 26, No. 1 (1975), pp 8-10, a thin film of a lithium niobate single crystal having an almost stoichiometric composition (Li/Nb=1) is formed on a substrate of a lithium tantalate single crystal. According to "J. Appln. Phys.", Vol. 70, No. 5 (1991), pp 2536-2541, a thin film of a lithium niobate single crystal is prepared on a substrate of lithium niobate doped with 5 mol % magnesium oxide by the liquid phase epitaxial process, while a lattice constant is varied by changing a ratio of Li.sub.2 O/Nb.sub.2 O.sub.5 of a melt. According to "J. Cryst. Growth", Vol 132 (1993), pp 48-60, a thin film of a lithium niobate single crystal having an almost stoichiometric composition is formed on a substrate of lithium niobate doped with 5 mol % magnesium oxide by the liquid phase epitaxial process. Further, according to Japanese patent application Laid-open No. 5-117,096, a thin film of a lithium niobate single crystal having a composition of Li/Nb=1 is formed by the liquid phase epitaxial process.
The film-forming process in the liquid phase epitaxial technique will be schematically explained with reference to a temperature schedule in FIG. 1. First, for example, lithium niobate (solute) and LiVO.sub.3 (melting medium) are charged and mixed together. At that time, the mixture is at room temperature. After the mixture is heated in a furnace, a melt-stirring rod is inserted into the mixture, so that the melt inside a crucible is stirred and fully fused (temperature: t.sub.1, time period: h.sub.1 -h.sub.2). The temperature of the melt is lowered slightly from t.sub.1 to t.sub.2 (time period: h.sub.2 -h.sub.3), and kept at this temperature t.sub.2 higher than a saturation temperature to uniformly melt lithium niobate and LiVO.sub.3 (time period: h.sub.3 -h.sub.4). The saturation temperature of the melt is determined by the charged composition of the solute and the solvent. Then, the melt is cooled to a temperature t.sub.3 lower than the saturation temperature (time period: h.sub.4 -h.sub.5) to keep the melt in a overcooled (supercooled) state (time period: h.sub.5 -h.sub.6). A substrate is contacted with the melt in the overcooled state, so that a film of a lithium niobate single crystal is grown by the liquid epitaxial process. Then, the temperature of the melt is lowered to room temperature (time period: h.sub.6 -h.sub.7).
An experiment for the production of a film of a single crystal of lithium niobate was carried out by using a conventional producing apparatus schematically shown in FIG. 2. That is, a heater 2 was buried in a wall of a furnace 1, and a crucible 11 was placed in the furnace 1. A melt 12 was charged into a crucible 11, and the crucible 11 was fixed on a table 13. A rotary shaft 5 was inserted through an opening 17 formed in an upper portion of the furnace 1, and a holding section 4 made of platinum was provided at a lower end of the rotary shaft 5. A substrate 3 of a lithium niobate single crystal was held by the holding section 4. The holding section 4 and the substrate 3 were rotated via the rotary shaft 5 and gears 6, 7 by driving a motor 8 in a direction of an arrow A. The gears 6, 7 and the motor 8 were attached onto an arm 9, and a projection 9a of the arm 9 was fitted to a lifter 10. First, a raw mixture was placed in the crucible 11, and heated inside the furnace. A stirring shaft (not shown) was descended to insert its tip into the crucible 11. The shaft was rotated, and the mixture was melted under stirring (temperature t.sub.1). Then, the stirring shaft was pulled up out of the furnace 1, and the melt was converted to an overcooled state. On the other hand, the rotary shaft 5 was gradually descended to make the substrate 3 contact with the melt in the overcooled state.
However, having examined this producing process, the present inventors have found the following problems. That is, in order to contact the substrate 3 of a lithium niobate single crystal with the melt 12, it is necessary to gradually descend the substrate at room temperature and contact it with the melt 12 in the overcooled state. However, it was frequently found that the lithium niobate single crystal substrate 3 was cracked or the crystallinity of the single crystal film was deteriorated.
Having examined causes for the above problems, the present inventors have reached the following conclusion. That is, since the melt 12 is ordinarily at a high temperature not less than 900.degree. C., there occurs a temperature gradient inside the furnace 1 that the temperature of the melt is higher at a lower portion. In addition, since the substrate 3 is almost horizontally held, the temperature of an under face of the substrate 3 becomes higher by heat radiation as the substrate is descended. In particular, since the substrate of an oxide type single crystal such as a lithium niobate single crystal has low heat conductivity, a relatively large temperature difference is likely to occur between the upper face and the lower face of the substrate 3. As a result, the substrate 3 was likely to be cracked due to difference in thermal expansion. In addition, since the lithium niobate is ferrodielectric, pyroelectricity is likely to occur due to this temperature difference, so that such a single crystal was greatly cracked by this pyroelectricity.
In order to prevent the above cracking of the substrate 3, the temperature difference between the upper face and the lower face of the substrate 3 may be reduced by decreasing the descending speed of the substrate 3. However, if a time period of descending the substrate 3 is prolonged, the time necessary for the formation of the single crystal film is extremely long so that productivity of the single crystal film largely drops. In addition, if the substrate-descending speed is remarkably reduced, the substrate was unlikely to be cracked. However, the crystallinity of the single crystal film was deteriorated. This is considered to occur due to the difference in thermal expansion and pyroelectrocity. The above problem similarly occurred when the substrate of the oxide type single crystal substrate was pulled up in the furnace after contacting the melt.